1. Field of the Invention
The invention lies in the field of the electro-optical measuring of distance. It relates to a method and a measuring device for measuring an absolute distance according to the generic term of the corresponding independent claims.
2. Description of Related Art
High-resolution distance measurements are taken by using instruments for the determination of a relative distance, e.g. laser interferometer, wherein a collimated laser beam travels from a measuring device to a reflecting target. The projected beam is superimposed upon the reflected beam received in the measuring device. As the distance changes, the intensity of the superimposed beams changes according to the interference of the two rays. Such variations of intensity are detected and registered by means of a counter. The distance variation is determined according to the number of intensity changes and the frequency of the laser light. Based upon a predetermined reference value, i.e. an absolute distance in a starting position, an absolute distance of other positions can, thus, also be determined. In order to measure the distance to a moving reflector or target, measuring devices are designed as trackers, i.e. the laser beam automatically follows the target by means of a rotating mirror. Elevation and azimuth of the laser beam are measured, enabling the determination of the target position in three dimensions. Based on this simple measuring principle, the position can be registered even at a target speed of, for example, up to 10 m/s.
The condition for such a method of determining a relative distance is that the beam between the measuring device and the target is not interrupted. If this does happen, the distance variations are no longer registered and the absolute distance between measuring device and target cannot be known. This absolute distance must therefore be determined or calibrated anew by other means. Such a combination of an absolute distance meter and an interferometer is revealed in the publication DE 195 42 490 C1.
Various methods of measuring an absolute distance are known, e.g. different variations of the Fizeau method, as also described and quoted in the DE 195 42 490 C1 mentioned above. To be suitable as a base value for an interferometer method, such a distance must be of similar accuracy, i.e. e.g. in the range of micrometers at a measuring distance of up to 100 meters.
Contrary to measuring a relative distance however, measuring an absolute distance at such a distance and accuracy requires a certain minimal length of measuring time during which the distance must not change. Therefore the target cannot be held manually by an operator but has to be placed on a steady support for calibration, which entails a time-consuming interruption in the measuring procedure.
WO 02/084327 A2 describes the method of measuring an absolute distance by means of laser light, wherein the ray is guided alternately along a measuring light path and a reference light path. The measuring light path extends along the distance to be measured and the reference light path lies within the measuring device. Thus, a distance variation of an internal reference light path is determined to compensate drift and temperature related changes.
An indirect determination of position by means of a tracker is described in WO 00/63645, wherein a position of a reference point of a measuring device is to be determined. The reference point is not visible from the tracker. A retro-reflector on the measuring device is moved along a known track, followed by the tracker. The position and orientation of the reference point can be determined from the measured positions of the retro-reflector and from the known geometry of the measuring device.